The present disclosure relates to portable communication devices, examples of which include mobile, or handheld, devices such as pagers, cellular phones, cellular smart-phones, wireless organizers, personal digital assistants, wirelessly enabled notebook computers, and the like; and more particularly to controlling the intensity of the wireless signals transmitted by such portable communication devices and the rate at which data are transmitted.
A wide variety of different types of portable wireless communication devices are on the market for communicating voice, data, images, and other forms of information. When being used, some of these devices are held against the ear of the user and some of the emitted radio frequency energy is absorbed by the user's body. A measurement of absorption of energy at a particular radio frequency is specified as a Specific Absorption Rate (SAR). As will be appreciated, the SAR value depends heavily upon the location of the transmitting antennas with respect to the body and the amount and the duration of the transmitted power. With a cell phone, for example, that is held against a person's ear, a greater intensity of radio frequency energy can be emitted by an antenna located near the bottom of the device positioned adjacent the jaw of the user as compared to when an antenna is located at the top of the device immediately adjacent to the ear.
Government agencies, such as the Federal Communication Commission (FCC) in the United States of America, have adopted limits for safe exposure to radio frequency (RF) energy. For example, the FCC limit for exposure from cellular telephones is a SAR level of 1.6 watts per kilogram (1.6 W/kg), which is referred to as a specific absorption rate limit.
Voice and data transmissions may employ a communication protocol in which the transmissions occur in a one millisecond transmission slot contained within a 20 millisecond frame, i.e., a given communication device transmits once every 20 milliseconds. When transmitting data, it is desirable to utilize as many of transmission slots in each frame as possible in order to send the data quickly. However, the more of the frame that is used, the greater the RF energy that is emitted and thus the specified SAR limit may be exceeded by the data transmission.
The transmissions may use different modulation and coding schemes (MCS). Such schemes commonly used by portable communication devices include quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM), each having several different coding rates at which the data are transmitted during each transmission slot. The data throughput, i.e., the average rate of successful message delivery over the communication channel, can be increased by changing the MCS to one that provides a higher coding rate. Nevertheless the different MCS's produce different specific absorption rates due to the different transmission power levels and thus the use of a particular MCS in a give situation may not be compliant with the specific absorption rate limit.
As a consequence, although the technology exists to increase the rate of data transfer, there is a concern that transmitting at a higher RF power and data rate could violate the SAR limit.